Any chance its the oil? Has about 4-5k on the oil in it.
No it's not the oil, now I have heard talk of 5W/40 getting better fuel economy than the 15W/40 but 4,000 miles on oil is still fine.
You can check all your intake boots to make sure they are all tight and no leaks from intercooler. I am guessing that your boost pressure is normal as usual?
Also check your plastic up pipe for any cracking at the bellow or flex area.
Is it running the same as usual?
Cold starts smooth?
Air filter ok?
You really need a way to read live data to see how the engine is actually runninng. A monitoring device can also keep you from having a major issue in regards to oil cooler failure can also see the health of the high pressure system
Low fuel pressure can also cause you a loss in fuel economy.
My list on must have bare minimum:
Live data monitoring Edge insight/ScanguageII ect.
Blue spring mod for fuel pressure regulator.
2006 F250 LARIAT CCSB 4X4
ARP Studs, O-Ringed Heads
X4 PHP 65 HP Custom Tow Tune
Atlas40 or Atlas80 Ficm Tune
CAT EC-1 Coolant, IPR Coolant filter
BPD Waterpump, BPD way of thinking
Fass-125 to a Adjustable Pressure Reg.
Turbo-back SS 4" pipe, Corsa Muffler -7" Tip
6.4 starter, DC Power 270 amp alt
Die Hard Platinum AGM all 2/0 Cables
Fabtech Radius Arms
7" lift w: Icon Progressive Coil Springs
2008 up Carli/Deaver rear springs
All riding on some Fatty-Daddy 38/15.50/18 Toyo's